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Salt Packs a Punch

In 420 B.C.E., the Greek historian Herodotus noted that salt was eating away the Egyptian pyramids. More than 2 millennia later, scientists are still trying to determine how salt wrecks rock and other porous building materials. Now, physicists have proved that salt crystals growing in nanometer-sized pores exert tremendous pressure on the pore walls. That "crystallization pressure" may be just the thing that takes chips off the old block.

Many researchers believe that salt damages stone when crystals form inside the stone's tiny pores. Water carries salt from the environment into a pore, and, as salt crystals precipitate out of the solution and expand, they run into the pore wall. If the natural properties of the salt and stone cause them to repel each other, the crystal will push against the stone with enormous force as it continues to grow. Or so the theory goes. No one has measured such crystallization pressure.

To measure it indirectly, Klaas Kopinga and colleagues at Eindhoven University of Technology in the Netherlands, took advantage of the fact that as the pressure in a crystal increases, the solubility of the salt in the surrounding water should also increase. That's because it's harder to add material to a crystal that's being squeezed. The researchers soaked porous glass in a solution of the salt sodium carbonate. Using nuclear magnetic resonance--the basic principle behind MRI scans-the researchers found that the salt was more soluble in waterlogged glass with pores 7 or 10 nanometers wide than it was in water alone, as they report in the 25 February issue of Physical Review Letters. From the increase in solubility, the researchers calculated that the pressures inside the pores reached as high as 13 megapascal, more than 4 times the tensile strength of typical building material.

"This is a wonderful paper demonstrating conclusively that we can see the crystallization pressure in a porous material," says Eric Doehne, a geologist at the Getty Conservation Institute in Los Angeles. George Scherer, a materials scientists at Princeton University in New Jersey, says that the technique "is quite brilliant and probably will be widely used" to study the interactions of other salts and materials. By understanding how salt ravages stone, researchers may be better able to combat its effects, Scherer says. But all agree that to fully explain the effects of salt, scientists will also have to account for the more complicated effects of repeated wetting and drying.